The present invention relates to a process for forming an electret nonwoven filter and products of such a process.
Nonwoven webs of electret fibers are typically formed of loosely associated fibers. The filters can be electrostatically charged prior to, during, or after, being formed in to a nonwoven web. For example, post-formation charging is described in U.S. Pat. No. 4,588,537 which charges a lofty nonwoven web formed by a variety of methods including carding and melt blowing. The webs are charged while under compression and then permitted to return to their original loft. This patent states that precharged fibers are preferably bipolar so that they tend to resist crimping which is avoided by pre-crimping when using the charging method of this patent.
Fibers can also be charged while they are being formed, as disclosed in Kubik et al., U.S. Pat. No. 4,215,682, where melt-blown fibers are bombarded by ions or electrons immediately after being extruded from the melt-blowing orifices. The fibers solidify extremely rapidly in the atmosphere and are collected as a semi-coherent mass of entangled microfibers. The webs are described as preferably open to provide a low pressure drop for liquid passing through the filter.
A particularly effective method of forming a nonwoven electret fiber filter is described in U.S. Reissue Pat. No. 30,782 (Van Turnout et al.). The electret fibers in this patent are formed from a corona charged film that is fibrillated to form the charged fibers. The charged fibers can then be formed into a nonwoven web by common methods such as carding or air laying. This charging method provides a particularly high density of injected charges. However, problems are encountered with forming webs from these precharged fibers. The fibers are generally quite large and uncrimped. They also have a resistance to bending. Due in part to these properties, the fibers resist formation into a uniform coherent web, particularly at low basis weights. This problem is partially addressed in U.S. Pat. No. 4,363,682, which proposes the use of such filibrated fiber webs in face mask applications. In order to provide a more coherent web, as well as one that resists shedding fibers, this patent proposes a post-embossing treatment. This post-embossing welds the outer surface fibers together allegedly providing a more coherent and comfortable web for use as a face mask. However, this treatment will also tend to result in a more condensed web, which would increase pressure loss over the filter.